Ironically, I need to give a bug report regarding a reverse of what happened to my fellow mod-user here: As Mali, playing on Epic game speeds and Corp & Monopoly game mode, I've just received *4* great merchants after building my cap suguba.
I wonder if the previous user confused the city state trait into thinking that each new suguba and not just the capital one gets merchants - leading to a fix that actually doubled the cap merchant gain.

Or maybe this is just a new bug. I hope this helps.

Thanks for your feedback, now it works also with the Malian Suguba

I just had a game when i was playing as the Mali, when i found this city-state. Naturally i tried to get Suzerainship as soon as possible, but when i was the Suzerain I did not get a great Merchant when i built my Suguba's. So I left the AI take over instead of wasting envoy's, but the AI started to get great merchants from the city state.
TL:DR- the Mali doesn't work properly with this city state.